Apparatus related to that of the present invention is described in U.S. Pat. No. 4,154,044. This patent describes apparatus for sealing cans comprising a lifting mechanism associated with a lifting plate; a closing plate spaced below and in spring biased cooperation with such lifting plate which can be moved in a sealed manner against the lower rim of a vacuum bell; and apparatus for lifting the lid of a can on such lifting plate mounted in the upper portion of such vacuum bell. Such lifting apparatus comprises a plate with a permanent magnet supported within the vacuum bell in a spring biased manner.
In operation the lifting device moves the lifting plate upward until the lid of the can engages the permanent magnet and the closing plate seals the vacuum bell closed. The lifting plate with the can is then lowered slightly with the vacuum bell remaining tightly closed and the lid engaged to the magnet. Air is then evacuated and, as needed, partially filled with protective gas. Following these acts, the lifting plate with the can is moved upward once again, until the lid contacts the can. The can is then pressed upward by the lifting mechanism compressing the rim of the lid into sealed engagement with such can. The vacuum bell is then filled with air and the lifting plate with the sealed can moved downward completing the operation. This apparatus has proved itself excellent as a semi-automatic apparatus; however, it is unsuitable for attaining extremely high outputs on the one hand, and for use when the cans are of various sizes on the other.
An apparatus of this kind, known from German Offenlegungsschrift (laid open patent application) No. 2,317,517 is provided with a total of three lifting devices designed as pneumatically chargeable lifting cylinders, one of which actuates a lower vacuum bell, a second actuates an upper vacuum bell, and a third actuates the pressure plate of a sealing apparatus. The operation is such that a can, provided with a lid is placed in a lower plate-shaped recess after the lower vacuum bell has descended, after which the lower vacuum bell is moved upward and the upper vacuum bell is moved downward until the vacuum chamber is enclosed. Then the pressure plate, provided with a permanent magnet, a pneumatically acting sucker or the like, moves against the cover and then returns, leaving the lid adhering to the pressure plate. Then the vacuum chamber is evacuated, after which the lid is pressed onto the can by means of the pressure plate. Then air is admitted to the vacuum chamber again and the two vacuum bells move apart, upward and downward, respectively.
Although this known apparatus operates well in theory, it has the significant disadvantage that a considerable amount of effort is required for the three lifting devices and the switches required to control them, this effort being justified only for fully automatic apparatus having a high efficiency which operates under a continuously full load. Furthermore, this apparatus is not suitable for being converted at low cost for use with cans of different sizes.